1. Field of the Invention
The present invention relates to a photoelectric converting device and a fabricating method therefor, and more specifically, to a photoelectric converting device which comprises a passivation film made of silicone resin directly covering a photoelectric converting part and to its method of fabrication.
2. Description of the Prior Art
These years attention has been directed to amorphous semiconductors, because they, as industrial materials, have excellent processing characteristics and mass-productivity and are low in cost. Further they have such a sensitive structure that their physical property constants can be continuously varied by controlling the composition of elements in forming the semiconductor material.
For example, since an image sensor employing a Schottky photoelectric converting element in which amorphous silicon is used as its photoconductive layer has a good light response characteristic (having a light/dark current ratio of more than 10.sup.4), it is effectively utilized as an excellent direct contact type image sensor.
FIGS. 1(a) and 1(b) respectively show cross sectional views of planar image sensors. More specifically, an image sensor 10 shown in FIG. 1(a) comprises opaque opposing electrodes 12 and 13 provided on a substrate 11 spaced by a predetermined distance, a photoconductive layer 14 formed on the electrodes 12 and 13, and a passivation film 15 formed on the photoconductive layer 14. The image sensor 10 receives a light from the side of the substrate 11 as shown by an arrow La. An image sensor 20 shown in FIG. 1(b), on the other hand, comprises a photoconductive layer 22 formed on a substrate 21, opaque opposing electrodes 23 and 24 provided on the layer 22 spaced by a predetermined distance, and a passivation film 25 covering the opposing electrodes 23 and 24. The sensor 20 receives a light from the side of the passivation film 25 as shown by an arrow Lb.
Referring to FIG. 1(c) and 1(d), there are shown, in section, image sensors of sandwich type. More particularly, an image sensor 30 shown in FIG. 1(c) comprises a transparent lower electrode 32 formed on a substrate 31, a photoconductive layer 33 formed on the electrode 32, an upper electrode 34 formed on the layer 33, and a passivation film 35 covering the upper electrode 34 and the layer 33. The image sensor 30 receives a light from the side of the substrate 31 as shown by an arrow Lc. On the other hand, an image sensor 40 shown in FIG. 1(d) comprises a lower electrode 42 formed on a substrate 41, a photoconductive layer 43 formed on the substrate 41 and lower electrode 42, a transparent upper electrode 44 formed on the layer 43, and a passivation film 45 covering the layer 43 and electrode 44. The image sensor 40 receives a light from a side of the passivation film 45 as shown by an arrow Ld.
Among the image sensors 10, 20, 30 and 40, the image sensors 10 and 30 which receive light from the substrate side requires that the substrate be made of a transparent material such as glass. The substrate also functions as a passivation film. The passivation film provided on the opposite side may be opaque and therefore the material and thickness of the passivation film is relatively widely selected.
On the other hand, the image sensors 20 and 40 receive light from the side of the passivation film and accordingly the passivation film must be transparent.
Usually, an inorganic film such as a silicon oxide (SiO.sub.x) film formed by the plasma CVD or an inorganic film coated with macromolecular resin such as polyimide resin is used as the passivation film. However, when silicon oxide film is used, the Schottky junction of the sensor is often destroyed during formation of the silicon oxide film due to the high temperature required during the film formation. Especially, in the case of a sensor of sandwich structure in which a photoconductive layer is held between upper and lower electrodes, these two electrodes are likely to be short-circuited in the plasma environment by the plasma CVD. Further, when the inorganic film coated with macromolecular resin is used as the passivation film, the photoelectric converting part may be sometimes destroyed during the formation of the passivation film because the curing temperature of the resin itself is as high as 350.degree.-400.degree. C. That is, when the lower electrode in the sandwich type is made of, for example, chrome (chrome is often employed), the sensor characteristics will be often deteriorated during the formation of the polyimide resin film because chrome starts to oxidize at 250.degree. C. Further, the polyimide resin does not have good optical characteristic. For example, the polymide resin absorbs lights whose wavelength is 480 nm or below. Furthermore, the low moisture and heat resistance properties in either case make it difficult to keep the image sensor stable with time without any deterioration of its characteristics.
Recently, there has been a tendency to elongate image sensors. Such elongated image sensors may be realized because an amorphous semiconductor having a large area can be easily formed. When the passivation film for the elongated image sensor is of a two-layer structure comprising an inorganic film and a polyimide resin film, it is difficult to uniformly form the inorganic film. Thus, the inorganic film is often subjected to cracking which reaches the polyimide resin film forming the upper layer, resulting in lowering of its moisture resistance.
As mentioned above, in either type of passivation film, it has been impossible to keep the image sensor stable for a long time without any deterioration of its characteristics.
In particular, when it is desirable to make such a photoelectric converting device formed on a large area substrate as an elongated image sensor, it has been impossible to make a passivation film which has good bonding and humidity resistance properties and can operate stably.